Program cartridges are at present used in many microcomputer systems. They were first introduced in the field of games computers to provide program data defining different games which can be played.
Essentially, a program cartridge comprises a read-only store mounted on a printed circuit board which includes connection pads for connection to a microcomputer through a socket mounted in the microcomputer. U.S. Pat. No. 1,595,156 (Asher and Hardy) shows the arrangement of a typical cartridge device. The cartridge read-only memory normally contains control code which is additional to control code contained in read-only memory in the microcomputer itself. Thus, in operation, the microcomputer is essentially controlled by the control code held internally and requests extra code for different applications from the cartridge read-only memory.
It has now been recognised that in certain conditions, it may be desirable to provide the base control code from a cartridge rather than the internal read-only memories. This enables complete re-configuration of the microcomputer by changing the base control code. It is to effect such change of control that the present invention is directed.